Fe-ni or fe-ni-co or fe-ni-co-cu alloy strip with improved cuttability

ABSTRACT

Strip made of an austenitic Fe—Ni alloy or Fe—Ni—Co alloy or Fe—Ni—Co—Cu alloy, the chemical composition of the alloy of which comprises, in % by weight: 30%≦Ni≦70%; 0%≦Cu+2×Co≦20%; 0%≦Mn+Cr&lt;5%; 0%≦W+2×Mo≦2%; 0%≦Ti+V+Nb+Al≦1%; 0.0005%≦B≦0.007%; the balance being iron and impurities such as C, S, P, O and N; the chemical composition being such that Fe+Ni+Cu+Co≧95%. The alloy has a cubic texture with a cubic texture index D c ≧7. Process for manufacturing a strip. Process for manufacturing a part by mechanical cutting.

[0001] The present invention relates to the manufacture of parts made of an alloy of the Fe—Ni or Fe—Ni—Co or Fe—Ni—Co—Cu type obtained by the precision mechanical cutting of blanks, which may possibly be drawn beforehand. These parts are in general used in miniature electrical or electronic components.

[0002] Many parts of small size, such as electron-gun parts for colour display cathode-ray tubes, or integrated-circuit support frames or leadframes, or parts for micromotors, are manufactured by the precision mechanical cutting of blanks which are possibly drawn, made of an alloy of the Fe—Ni or Fe—Ni—Co type containing about 30% to 70% nickel. The quality of the cutting is very important for this type of part, particularly in order to prevent the presence of burrs.

[0003] The blanks from which the parts are cut are taken from generally isotropic or slightly textured strip obtained by cold-rolling and annealing. In the case of flat or almost flat parts, that is to say those obtained without appreciable plastic deformation of the blank, the strip is often used in the work-hardened state so as to have a higher hardness and a lower ductility than strip obtained directly after annealing. This higher hardness and lower ductility favour mechanical cutting. In contrast, when the drawing is substantial, the strip is used in the annealed state so as to have a high ductility and the ability to undergo extensive plastic deformation. In this case, the cutting operation, which is the drilling of a hole, is sometimes preceded by local work-hardening intended to reduce the ductility of the metal along the cutting line. However, both in the case of slightly drawn parts and parts which are highly drawn, the quality of the cutting is often insufficient, and this leads to an appreciable proportion of the cut parts being scrapped.

[0004] To improve the quality of the cutting, it is known to add small amounts of alloying elements such as molybdenum or niobium to the alloy, or to maintain very low amounts of residual elements such as carbon, sulphur, phosphorus or oxygen which may form inclusions. However, these means are insufficient and the mechanical cutting capability of the Fe—Ni or Fe—Ni—Co or Fe—Ni—Co—Cu alloys is deemed to be very inferior compared with, for example, that of 305 stainless steel.

[0005] It is an object of the present invention to remedy this drawback by providing a means for improving the cuttability of Fe—Ni or Fe—Ni—Co or Fe—Ni—Co—Cu type alloys used in thin strip form for the manufacture by precision mechanical cutting of parts used especially in electronic or electrical equipment.

[0006] For this purpose, the subject of the invention is a strip made of an austenitic Fe—Ni alloy or Fe—Ni—Co alloy or Fe—Ni—Co—Cu alloy in which the chemical composition of the alloy comprises, in % by weight:

30%≦Ni≦70%

0%≦Cu+2×Co≦20%

0%≦Mn+Cr≦5%

0%≦W+2×Mo≦2%

0%≦Ti+V+Nb+Al≦1%

0.0005%≦B≦0.007%

[0007] the balance being iron and impurities such as C, S, P, O and N; the chemical composition being such that Fe+Ni+Cu+Co≧95%. In addition, the alloy has a cubic texture with a cubic texture index D_(c)≧7.

[0008] As a preference, separately or in combination, the boron content is between 0.0007% and 0.004%, the D_(c) index is greater than 10, the carbon content is less than or equal to 0.05%, the sulphur content is less than or equal to 0.01%, and better still less than or equal to 0.007% and the oxygen content is less than 0.005%.

[0009] The invention also relates to a process for manufacturing a strip made of an Fe—Ni or Fe—Ni—Co or Fe—Ni—Co—Cu alloy in which:

[0010] a strip made of an alloy whose chemical composition is defined above is manufactured by cold-rolling with a deformation ratio of greater than 80%;

[0011] a fine-grain recrystallization annealing operation is carried out on the strip;

[0012] and, optionally, a complementary cold-rolling operation is carried out with a deformation ratio of less than 40%.

[0013] Finally, the invention relates to a process for manufacturing a part by mechanical cutting or by mechanical cutting and drawing, in which a blank is taken from a strip according to the invention and at least one mechanical cutting operation and optionally at least one drawing operation are carried out on the blank, it being possible for the at least one drawing operation to be carried out before or after the at least one mechanical cutting operation. The part is, for example, an electron-gun part with a hole through which the electrons pass. The part may also be a leadframe with connection leads. The part may also be a magnetic core of a micromotor or transformer. This list of applications is not limiting.

[0014] The invention will now be described in greater detail with regard to the appended figure and illustrated by examples.

[0015]FIG. 1-a represents, in schematic cross section, a strip in which a hole has been drilled by mechanical cutting, showing a cut surface corresponding to poor cuttability.

[0016]FIG. 1-b represents, in schematic cross section, a strip in which a hole has been drilled by mechanical cutting, having a cut surface corresponding to acceptable cuttability.

[0017]FIG. 1-c represents, in schematic cross section, a strip in which a hole has been drilled by mechanical cutting, having a cut surface corresponding to good cuttability.

[0018] The strip according to the invention is thin, cold-rolled strip (with a thickness in general of less than 1.5 mm) made of an alloy of the Fe—Ni or Fe—Ni—Co or Fe—Ni—Co—Cu type which are known per se in their most general form. In this type of alloy, the nickel, or the cobalt, which is a substitute for nickel, allows properties such as the thermal expansion coefficient or the magnetic permeability to be adjusted. The nickel content is between 30% and 70%; the copper and cobalt contents are such that the sum Cu+2Co is less than or equal to 20%, these two elements being optional. The balance is essentially iron, impurities, such as carbon, sulphur, phosphorus, oxygen and nitrogen, and possibly complementary alloying elements, such as manganese, chromium, tungsten, molybdenum, titanium, vanadium, niobium and aluminium. However, the iron, nickel, copper and cobalt contents must be such that: Fe+Ni+Cu+Co≧95%. The contents of the alloying elements must be such that: Mn+Cr<5%, W+2Mo≦2% and Ti+V+Nb+Al≦1%. Certain impurities, such as carbon, sulphur and oxygen which are in the form of inclusions, may be desirable in small amounts since they have a favourable effect on cuttability. Nevertheless, the carbon content, must, preferably, remain less than 0.05%, the sulphur content must preferably remain less than 0.01%, and better still less than 0.007%, and the oxygen content must, preferably, remain less that 0.005%.

[0019] Furthermore, the alloy contains from 0.0005% to 0.007%, and preferably from 0.0007% to 0.004%, boron and has a (001)<100> cubic texture characterized by a cubic texture index D_(c) of greater than 7, and preferably greater than 10. This is because the inventors have found, surprisingly, that an addition of boron combined with a highly pronounced cubic texture very substantially improves the mechanical cutting capability of alloys of the Fe—Ni or Fe—Ni—Co or Fe—Ni—Co—Cu type.

[0020] The cubic texture index D_(c) is the ratio, I_(cubic)/I_(isotropic), of the maximum reflected X-ray intensities, measured on a (111) pole figure at a point located at 54°44′ from the centre of the figure and along the line at 45° to the rolling direction for a specimen of the strip to be characterized on the one hand and for an isotropic specimen on the other.

[0021] The degree of texture of a strip may also be evaluated simply but approximately by a drawing test by measuring the drawing ears. This method can be used only to characterize a sufficiently ductile metal. To use this test, it is possible, for example, to start with a 60 mm diameter disc, to draw this disc so as to form a cup 33 mm in diameter and 19 mm in height on average. The difference in height between the highest points and the lowest points of the upper edge is then measured. If this height difference is less than 0.3 mm, the strip is isotropic or has very little texture; if this difference is greater than 1.5 mm, the strip has a highly pronounced texture.

[0022] To obtain a strip of alloy having a pronounced cubic texture, the alloy is smelted, cast and hot-rolled in a manner known per se so as to obtain a hot strip of sufficient thickness to allow a cold strip having the desired thickness to be obtained by cold-rolling with a reduction ratio of greater than 80%, and better still greater than 90%. The thickness of the hot-rolled strip may, for example, be 5 mm. The cold rolling must be carried out without intermediate annealing, but may be preceded by an annealing step. This is the case, in particular, when, on account of the thickness of the hot strip and the intended thickness for the cold strip, it is necessary to carry out several successive cold-rolling passes. The final cold-rolling pass (with a reduction ratio of greater than 80%) is followed by a recrystallization annealing step generally carried out in a tunnel furnace in a protective atmosphere consisting, for example, of a mixture of hydrogen and nitrogen with a dew point below −40° C. The temperature of the oven, about 1000° C., must be sufficient to obtain fine-grain recrystallization, but not too high in order to prevent undesirable coarse-grain secondary recrystallization. The duration of the annealing step is in general around one minute. Those skilled in the art will know how to adapt, on a case-by-case basis, the precise annealing conditions so as to obtain fine-grain recrystallization while avoiding secondary recrystallization.

[0023] Optionally, and so as to increase the hardness of the strip, the recrystallization annealing may be followed by complementary cold-rolling with a reduction ratio of less than 50° or better still less than 30°, in order not to excessively degrade the initial cubic texture. When the reduction ratio of the complementary cold-rolling is less than 10%, a cold strip, softened or slightly work-hardened, having a pronounced cubic texture is obtained. When the reduction ratio of the complementary cold rolling is greater than 10%, a work-hardened cold strip with a pronounced cubic texture is obtained.

[0024] Given the thickness of the hot strip, the cold-rolled strips obtained have a thickness generally of less than 0.5 mm.

[0025] To manufacture a part according to the invention from a strip having a cubic texture index of greater than 7, or better still, greater than 10, or even better greater than 15, a blank is cut by mechanical cutting in a manner known per se. The blank may either be the finished part, which is then flat, or a preform. The preform may be formed by drawing and then cut again by mechanical cutting. This cutting may, for example, be a drilling operation. This cutting operation may be preceded by a local work-hardening step.

[0026] When the part is flat or slightly deformed by drawing, that is to say when the deformation ratio produced by the drawing is less than 20%, the strip can be used after a complementary cold-rolling operation with a reduction ratio of between 10% and 30%, or even 50%. This is the case, for example, for flat parts for electron guns of colour display cathode-ray tubes, or for leadframes having connection leads, or for rotors or stators of electric micromotors.

[0027] When the part is highly deformed, by drawing or by bending or by a local thickness reduction, that is to say with deformation ratios of greater than 20%, the strip is used in the softened or slightly work-hardened state, that is to say without complementary cold-rolling or with complementary cold-rolling having a reduction ratio of less than 10%. This is the case, for example, for certain electron-gun parts for colour display cathode-ray tubes.

[0028] The quality of the cutting is assessed by the cut surface, which comprises a sheared region and a torn region. The line of demarcation between these two regions must be regular and located at approximately mid-thickness. There must not be any burrs.

[0029] Three cut surfaces are shown in FIGS. 1a, 1 b and 1 c. These surfaces are those observed around a hole 1 a, 1 b and 1 c, drilled in a strip 2 a, 2 b and 2 c by punching. Only one half of each hole is shown after sectioning the strips in a plane passing through the axis of the holes. The walls 3 a, 3 b and 3 c of the holes each have a sheared region 4 a, 4 b and 4 c and a torn region 5 a, 5 b and 5 c.

[0030]FIG. 1a corresponds to a strip of an alloy having poor cuttability. The sheared region 3 a corresponds to most of the thickness and it terminates near the bottom, in substantial burrs 6 a.

[0031]FIG. 1b corresponds to a strip of an alloy having a cuttability which is just acceptable. The sheared region 3 b corresponds roughly to half the thickness and terminates, near the bottom, in a few burrs 6 b.

[0032]FIG. 1c corresponds to a strip of an alloy having excellent cuttability. The sheared region 3 c corresponds to roughly half the thickness and has no burrs.

[0033] These figures are given merely by way of indication. Those skilled in the art will know how to assess in each case the quality of the cutting in accordance with more precise criteria than that which can be deduced directly from these figures.

[0034] Apart from the observation of these cut surfaces, the quality of the cutting can also be assessed by the geometrical quality of the parts obtained. When the cutting is the drilling of a round hole, the assessment of the cutting quality takes into account how circular the hole is.

[0035] In general, the observations needed to evaluate the quality of the cut parts are made with a magnification of between ×10 and ×50.

[0036] By way of examples and comparisons, hot-rolled strips 4.5 mm in thickness made of an FeNi42 alloy, the chemical compositions of which in % by weight are given in Table 1, were manufactured. Ref. Ni Mn Si C S P Al B N O Fe PV408 40.8 0.45 0.07 0.005 <0.0005 0.003 <0.005 <0.0005 0.002 0.0025 Bal. PV588 41.1 0.40 0.10 0.004 <0.0005 0.004 <0.005 0.0038 0.002 0.0025 Bal. PW075 40.6 0.43 0.12 0.009 0.0032 0.003 <0.005 0.0018 0.002 0.0035 Bal.

[0037] Alloys PV588 and PW075 have compositions in accordance with the invention while alloy PV408 is given by way of comparison.

[0038] Cold strips were produced from these hot strips according to 6 different manufacturing schemes, denoted A, B, C, D, E and F, comprising: a first cold-rolling pass with a deformation ratio DEF1, a tunnel-furnace recrystallization annealing step and a second cold-rolling pass with a deformation ratio DEF2. The first deformation was in certain cases preceded by a preliminary deformation of the hot-rolled strip followed by a recrystallization annealing step in order to adjust the thickness to the desired value.

[0039] The deformation ratios for each scheme, together with the hardnesses Hv and elongations at break A% obtained (which are identical for the three alloys), are given in Table 2. TABLE 2 Scheme A B C D E F DEF1 68% 50% 88% 91% 96% 96% DEF2 60% 23% 23% 20% 20% 44% Hv 230     205     205     205     205     220     A %  1%  5%  5%  5%  5%  2%

[0040] For each of these schemes, the cubic texture indices D_(c) obtained for each of the alloys are given in Table 3. TABLE 3 A B C D E F PV408 D_(c) = 1 D_(c) = 1 nd nd D_(c) = 25 nd PV588 D_(c) = 1 D_(c) = 1 nd D_(c) = 10 D_(c) = 25 D_(c) = 7  PW075 D_(c) = 1 D_(c) = 1 D_(c) = 10 D_(c) = 25 D_(c) = 45 D_(c) = 12

[0041] To obtain a cubic texture index D_(c) of greater than 10, these results show that the deformation ratio DEF1 must be high and preferably greater than 80% and the deformation ratio DEF2 must not be too high.

[0042] The cold-rolled strips made of PV588 alloy obtained using schemes D, E and F and the strips made of PWO75 alloy obtained using schemes C, D, E and F correspond to the invention. The other ones are given by way of comparison.

[0043] Leadframes were produced, by mechanical cutting, from the 0.25 mm thick strips corresponding to the three alloys and to manufacturing scheme D. The PV588 and PW075 alloys, containing boron in accordance with the invention, gave 100% good parts. On the other hand, the PV408 alloy gave poor parts with significant burring.

[0044] Three strips 0.4 mm in thickness were manufactured from the PW075 alloy (containing boron in accordance with the invention) using schemes A, B and C respectively, from which batches of flat parts were cut, these parts having a circular hole 0.5 mm in diameter obtained by punching.

[0045] In the batch corresponding to scheme A (comparison), only 24% of the parts were good and in the batch corresponding to scheme B (comparison) only 53% of the parts were good. In contrast, 100% of the parts of the batch corresponding to scheme C (invention) were good. 

1. Strip made of an austenitic Fe—Ni alloy or Fe—Ni—Co alloy or Fe—Ni—Co—Cu alloy, characterized in that the chemical composition of the alloy comprises, in % by weight: 30%≦Ni≦70% 0%≦Cu+2×Co≦20% 0%≦Mn+Cr<5% 0%≦W+2×Mo≦2% 0%≦Ti+V+Nb+Al≦1% 0.0005%≦B≦0.007% the balance being iron and impurities such as C, S, P, O and N; the chemical composition being such that Fe+Ni+Cu+Co≧95%, and in that the alloy has a cubic texture with a cubic texture index D_(c)≧7.
 2. Strip according to claim 1, characterized in that: 0.0007%≦B≦0.004%.
 3. Strip according to claim 1 or claim 2, characterized in that D_(c)>10.
 4. Strip according to any one of claims 1 to 3, characterized in that the carbon content of the alloy is less than or equal to 0.05%.
 5. Strip according to any one of claims 1 to 4, characterized in that the sulphur content of the alloy is less than or equal to 0.01%.
 6. Strip according to claim 5, characterized in that the sulphur content of the alloy is less than or equal to 0.007%.
 7. Strip according to any one of claims 1 to 6, characterized in that the oxygen content is less than 0.005%.
 8. Process for manufacturing a strip according to any one of claims 1 to 7, characterized in that: a strip made of an alloy whose composition is defined by claims 1 to 7 is manufactured by cold-rolling with a deformation ratio of greater than 80%; a fine-grain recrystallization annealing operation is carried out on the strip; and, optionally, a complementary cold-rolling operation is carried out with a deformation ratio of less than 40%.
 9. Process for manufacturing a part by mechanical cutting or by mechanical cutting and drawing, characterized in that: a blank is taken from a strip according to any one of claims 1 to 7; and at least one mechanical cutting operation and optionally at least one drawing operation are carried out on the blank, it being possible for the at least one drawing operation to be carried out before or after the at least one mechanical cutting operation.
 10. Process according to claim 9, characterized in that the part is an electron-gun part with a hole through which the electrons pass, or in that the part is a leadframe with connection leads, or in that the part is a magnetic core of a micromotor or transformer. 